1. Field of the Invention
The present invention generally relates to a positive electrode support material in electrochemical cells containing nonaqueous electrolytes and, more specifically, to nickel-based alloys as positive electrode current collector materials.
2. Prior Art
Solid cathode, liquid organic electrolyte, alkali metal anode electrochemical cells or batteries are used in applications ranging from power sources for implantable medical devices to down-hole instrumentation in oil/gas well drilling. Typically, the cell is comprised of a casing housing a positive electrode comprised of a cathode active material, material to enhance conductivity, a binder material, and a current collector material; a negative electrode comprised of active material such as an alkali metal and a current collector material; a nonaqueous electrolyte solution that includes an alkali metal salt and an organic solvent system; and a separator material isolating the electrodes from each other. Such a cell is described in greater detail in U.S. Pat. No. 4,830,940 to Keister et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
The positive electrode current collector serves several functions. First, it acts as a support for the cathode active material. Secondly, the positive current collector conducts the flow of electrons between the cathode active material and the positive cell terminal. Consequently, the material selected for this function affects the longevity and performance of the electrochemical cell into which it is fabricated. For one, the positive electrode current collector must maintain chemical stability and mechanical integrity in corrosive electrolytes throughout the anticipated useful life of the cell. In addition, as applications become more demanding on electrochemical cells containing nonaqueous electrolytes (including increased shelf life and extended long term performance), the availability of corrosion resistant materials suitable for these applications becomes more limited. For example, the availability of materials capable of operating or maintaining chemical stability at elevated temperatures is limited. Elevated temperatures may be encountered either during storage or under operating conditions (for example, down-hole in well drilling), or during autoclave sterilization of an implantable medical device powered by the electrochemical cell (Thiebolt III and Takeuchi, 1989, Progress in Batteries & Solar Cells 8:122-125).
In that respect, the prior art has developed various corrosion resistant materials useful for positive electrode current collectors. However, certain of these materials corrode when exposed to elevated temperatures of about 72° C. or higher, or when exposed to operating conditions in aggressive cell environments that can compromise surface passivity. Also, at elevated temperatures the chemical integrity of the positive electrode current collector may depend on the specific cathode active material. One particularly vexing combination is when fluorinated carbon (CFx) is contacted to a current collector of titanium. It is known that titanium reacts with species present within the internal cell environment to undesirably increase cell impedance by fluorination and excessive passivation of the current collector interface (Fateev, S. A., Denisova, O. O., I. P. Monakhova et al., Zashchita Metallov, Vol. 24, No. 2, pp. 284-287, 1988, transl.). The kinetics of this process are temperature dependent. At elevated temperatures, excessive passivation may occur quite rapidly (for example, at 100° C., the reaction requires less than 10 days).
Other current collector alloys used to fabricate positive electrode current collectors have been described in the art. For example, highly alloyed chromium-containing stainless steel materials are described in Japanese patent publications Nos. 18647 and 15067. However, the ferritic stainless steel material disclosed in publication No. 15067 requires costly melting procedures, such as vacuum melting, to limit the alloy to the cited carbon and nitrogen levels.
Highly alloyed nickel-containing ferritic stainless steels, which provide superior corrosion resistance, particularly, when elevated temperature storage and performance is required, are disclosed in U.S. Pat. No. 5,114,810 to Frysz et al. This patent is assigned to the assignee of the present invention and incorporated herein by reference. Frysz et al. disclose alloyed nickel-containing ferritic materials having less than 10% nickel, by weight. In addition to the relatively small quantity of nickel, the alloy contains, by weight, 28% to 30% chromium and 3.5% to 4.2% molybdenum; or 27% to 29% chromium and 2% to 3% molybdenum. In some battery environments, there may be other alloy materials that provide superior performance. In that respect, use of such alloyed low nickel ferritic stainless steels is limited in several respects. Chief among them is that the alloy is not readily available in thicknesses typically required for use as a current collector, and developing a commercial source has proven difficult. Current collectors are preferably thin to permit increased volumetric and gravimetric energy density, as well as to permit increased surface area per volume for rapid discharge at high current densities.
Therefore, the present invention is directed to providing a positive electrode current collector material that exhibits chemical compatibility with aggressive cell environments; provides high corrosion resistance, but does not develop excessive passivation in the presence of fluorinated active materials such as fluorinated carbons, and thereby maintains its inherent high interfacial conductivity; provides resistance to surface activation by material handling or mechanical means; and can be manufactured in the required form and thicknesses.
Nickel-based alloys according to the present invention offer the characteristics required of such positive current collectors. This class of metals also offers other advantages, especially when used in cells for implantable medical devices. Typically, the power source of an implantable medical device contains current collectors of wrought metal stock in sheet or foil form made by convenient and economical chemical milling/photoetching processes. In contrast to the previously described relatively costly prior art fabrication processes for high chromium ferritic alloys (Japanese patent publication Nos. 18647 and 15067), chemical milling/photo etching these processes readily fabricate the present nickel-based alloy current collectors.
Even in the family of nickel-based alloys, however, selection is limited. Certain elemental constituents, especially copper, molybdenum and tungsten, are of vital importance in maximizing corrosion resistance. Also, chromium improves the formation of corrosion resistant passive surface films in the presence of oxygen. Silicon may also promote protective oxide formation at high corrosion potentials. Thus, the total amount of copper, silicon, chromium, molybdenum and/or tungsten present in a particular nickel-based alloy is a primary determinant to the suitability of that alloy as a current collector. Consequently, there are only a handful of acceptable compositions among available metals and alloys that remain practically corrosion-free in certain demanding cell environments; high chromium ferritic stainless steels are one class and selected nickel-based alloys are another.